LLC converters are a form of series resonant converters having a series resonant circuit including a transformer primary winding in which a switching circuit is used to alternately couple a switching node of a resonant circuit or tank circuit to a positive supply node or a ground node to provide an alternating resonant current flow through the transformer primary winding. A secondary circuit, such as a rectifier, provides an output voltage to drive a load, with the switching circuit operation being adjusted to regulate the output voltage. Series resonant converters can operate in a so-called inductive mode or in a capacitive mode depending on the operating frequency and impedances of the resonant circuit and the secondary circuit, with the impact of the output rectifier and any associated filter circuitry being reflected back into the primary resonant circuit through the turns ratio of the transformer, whereby the load impedance including the secondary circuitry is essentially in series with the resonant circuit. Frequency control of the switching circuit can be used to regulate an output condition (e.g., output voltage, output current, etc.) based on one or more feedback signals, where the resonant circuit impedance is minimized at a resonant frequency. LLC converters provide a resonant inductance, typically an inductor connected in series with the transformer primary winding and a resonant circuit capacitance, as well as a magnetizing inductor in the series circuit, with LLC resonant converters typically having two resonant frequencies. Operation at the higher resonant frequency advantageously facilitates regulation of a wide load range with minimal frequency adjustment.
LLC converters are preferably designed to operate in an inductive region with the resonant circuit current lagging the voltage. The lagging current operation in the inductive region facilitates zero-voltage-switching (ZVS) on the MOSFETs or other switching devices used in the series resonant converter, thereby enhancing system efficiency while mitigating switch degradation. However, certain operating conditions may change the resonant circuit operation toward the capacitive region, thereby inhibiting zero voltage or near-zero voltage switching operation. As a result, operation in the capacitive region is undesirable, and may lead to current reversal stresses on the switching components. Although operating frequency, resonant circuit component values and other system design parameters can be tailored to help prevent or minimize operation in the capacitive region, input voltage brownout or other drops in input voltage and/or output current overload conditions may cause a circuit to operate in the capacitive region, which can lead to undue stress on the switching devices. Also, certain circuit configurations may be subject to capacitive mode operation at startup before resonant circuit capacitors are fully charged to a steady state value. Thus, capacitive region operation reduces system efficiency, and may in some cases lead to degradation or damage to the switching devices. Accordingly, a need remains for improved control apparatus and techniques to promote zero voltage switching or near-zero voltage switching and mitigate operation in the capacitive region for LLC and other series resonant converters.